Image display projectors generate display images and project them onto display screens, typically for viewing by multiple persons or viewers. The display images are formed and projected typically by passing light from a high-intensity light source through an image-forming medium such as a translucent film or liquid crystal display (LCD).
FIG. 1 is a diagrammatic side view diagram of a conventional prior art illumination system 10 for a liquid crystal display (LCD) projector. Illumination system 10 includes a light source 12, such as a high-intensity tungsten-halogen quartz lamp, that is positioned in spaced apart relation between a light-collecting reflector 14 and a condenser lens 16.
Reflector 14 and condenser lens 16 cooperate to collect the light generated by light source 12 for use by the projector.
Light source 12 is spaced-apart from condenser lens 16 by a distance 18 of between 4 and 12 mm. Spacing 18 is selected according to at least two conflicting design constraints. Light source 12 preferably is positioned as close as possible to condenser lens 16 to maximize the amount of light condenser lens 16 receives from light source 12 through collection angel X. Conversely, spacing 18 preferably is maximized to reduce the amount of heat transferred from light source 12 to condenser lens 16.
The conventional 400 watt tungsten-halogen quartz lamp used in a liquid crystal display projectors has a bulb surface temperature of up to about 700.degree. C. Heat from light source 12 typically is considered to cause heat gradients within condenser lens 16. The heat gradients cause regions of condenser lens 16 to expand differentially relative to other regions, thereby introducing stress changes that can cause breakage in condenser lens 16. Increasing the spacing between light source 12 and condenser lens 16 reduces the heat gradients.
Some projectors use "arc" forms of lamps such as metal halide and xenon lamps. These lamps provide a special case of heating gradient problems in condenser systems. Arc lamps have overall small physical sizes and relatively small, intense sources with relatively high ultraviolet (UV) light outputs. The glass types commonly used in condenser lenses generally absorb UV light. As a consequence, most condenser lenses used with arc lamps overheat in a small area where the light flux is concentrated. Since the small physical size promotes close spacing the small sources create high localized flux, the overheating caused by UV light absorption can cause breakage of condenser lenses.
Increasing the spacing between light source 12 and condenser lens 16 passively reduces the temperatures within condenser lens 16. Conventional projectors also actively cool condenser lens 16 by directing an airflow 20 with a fan 22 transversely across light source 12, its base, and condenser lens 16. The airflow is transverse relative the optic axis 20 of the projector.
However, tungsten halogen and metal halide lamps require high envelope or bulb temperature in order to maximize service life. Conventional active cooling of condenser lens 16 can over-cool the envelope and shorten the service life of such lamps. Accordingly, the cooling of condenser lens 16 conflicts with maintaining a high bulb temperature, and typically results in less than optimal temperature conditions for both light source 12 and condenser lens 16.
Another consequence of the conventional spacing between light source 12 and condenser lens 16 is that the spacing allows light generated by light source 12 to escape condenser lens 16 and not be usable for projecting display images. The collection angle .alpha. decreases with increases in distance 18. This lost light reduces the brightness of the LCD projector.
A further consequence of the conventional spacing between light source 12 and condenser lens 16 is that relatively small movements or accelerations of the LCD projector, such as occur when it is moved or transported, can cause light source 12 to move relative to condenser lens 16. Such movement of light source 12 can cause light it generates to be misdirected and unusable for projecting display images.